Employee Benefit Plans - Narrative (Details) - USD ($) $ in Millions	12 Months Ended
	Aug. 31, 2019	Aug. 25, 2018	Aug. 26, 2017
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Contributions charged to expense under the plan	$ 19.7	$ 18.1	$ 15.0
SERP
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Amounts charged to expense related to the plan	2.1	2.1	2.5
Accumulated benefit obligations	27.4	21.7
UniFirst Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Amounts charged to expense related to the plan	$ 0.3	0.3	0.5
Minimum annual interest rate	3.25%
Textilease Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Amounts charged to expense related to the plan	$ 0.5	0.2	$ 0.2
Minimum annual interest rate	1.50%
Pension Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Accumulated benefit obligations	$ 5.6	$ 7.4